UCL Ear Institute

Bizley Lab

Bizley Lab

Our research seeks to link the patterns of neural
activity in auditory cortex to our perception of the world around us. While
sounds within an environment, such as a person talking, may be clearly
intelligible at their source, noisy and reverberant listening conditions
often combine to degrade the intelligibility of the sound wave arriving at the
ear. The sound wave that arrives at the ear results from the combination of all
of the different sounds that exist in our surroundings. This complex sound wave
is decomposed into individual frequency components by the cochlea and transmitted
via the auditory nerve to the auditory brain via the auditory nerve. The
challenge for the brain is then to separate different sound sources from one
another in order that they can be identified or understood - a process also known as auditory scene
analysis. The brain must regroup the
sound elements that arose from each individual source. Different sources can be
desribed according to their characteristics - they might have a particular
pitch, or a characteristic timbre. Our research falls into three broad themes:

We might expect to see neurons in auditory cortex whose
responses match our own perception so that they could perhaps represent a
particular sound pitch irrespective of whether it is played by a violin or
piano, to our left or right. However, most neurons in auditory cortex represent
multiple features of a sound, that is their activity is often modulated by more
than one stimulus dimension. Information may be 'multiplexed'

within single neurons since it is possible to extract
information about different stimulus parameters from single neurons at
different time points in the neural response. Moreover, responses of ensembles
of broadly tuned neurons (none of which alone demonstrate invariant

encoding) can be decoded to provide estimates of a
stimulus that are robust to changes in irrelevant stimulus features. However,
we know little about how sensitivity to particular sound features arises, or
what sort of neural representation underlies our perception. Current work is
exploring (i) how spectral timbre, which determines vowel identity in speech,
is extracted by auditory neurons and (ii) how auditory space and in particular
the relative spatial locations of sound sources are represented in auditory
cortex.

Sound identification - and speech comprehension - require
that we are able to generalise across highly variable acoustic stimuli. For
example we are able to recognize vowel sounds whether they are spoken by a high
or low pitch voice, across accents and genders and in noisy environments. We
seek to understand how such invariance, i.e. the ability to maintain perceptual
constancy in the face of identity preserving transformations, arises within
auditory cortex. We examine neural and behavioural discrimination of sound
features - such as spectral timbre - across varied listening conditions including
in the presence of noise. By recording populations of neurons across multiple
auditory areas we hope to track the emergence of invariant representations.

We often unconsciously
rely on what we see to make sense of what we are hearing (and vice versa). For
example, we look at lip movements when we are trying to listen to what someone
is saying in a noisy situation.

Traditionally information from different sensory systems,
such as the eye and the ear, were thought to be processed independently within
the brain. Recent studies have revealed that even at the earliest stages of
sensory cortex there is considerable cross-talk between sensory modalities. For
example, in the auditory cortex a sizable proportion of nerve cells are
sensitive to visual stimulation. Why should neurons in auditory cortex be
responding to light? A possible explanation is that visual inputs might
increase the spatial acuity of auditory neurons.

Another is that visual inputs might help to parse the
auditory scene into its constituent sources.

Our research methods combine human and non-human
psychophysics, computational modelling and behavioural neurophysiology. Since
attentional state, behavioural context and even the presence of visual stimuli
can modulate or drive activity in auditory cortex, we believe that auditory
cortical neurons should not be seen simply as static filters tuned to detect
particular acoustic features and that visualising neural and behavioural
sensitivity simultaneously is key to understanding how neurons in auditory
cortex support sound perception.